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Saturday, August 7, 2010

Wilderness Navigation: Magnetic Declination

Magnetic declination; the thing which we never want to talk about because it gives us headaches! It is unfortunately, a very important part of wilderness navigation, as it can considerably alter our course of travel over a significant distance. I will try to offer a very simple way to look at it.

What is magnetic declination:

Magnetic declination is the difference between the place where the needle on your compass points (magnetic North), and the actual or true North direction in a particular area. That’s right, the compass does not always point directly North. Depending on the area of the globe where you are located, the difference can be quite significant. Failing to compensate for that difference can have important implications to navigation.

How do I find out the magnetic declination in my area:

Fortunately, the nice people at the National Geophysical Data Center of the National Oceanic and Atmospheric Administrations (NOAA) have been kind enough to create a magnetic declination calculator for us. You can go to the website here: http://www.ngdc.noaa.gov/geomagmodels/Declination.jsp, punch in either your longitude and latitude, or just your zip code, and you will get the magnetic declination in your area.

What do I do with this number:

Here is the part that throws most people off, just because of the terminology used. Before we start, remember this:

Positive declination is the same as Easterly declination. (10 degrees is the same as 10 E) Negative declination is the same as Westerly declination. (-10 degrees is the same as 10 W)

Positive or Easterly declination means that the needle of the compass will point to the East (or clockwise) of true North by however many degrees you got from the above calculation. The first diagram shows 10 E or 10 degrees on a compass. Wherever the needle of the compass points, it is 10 degrees East (or clockwise) of true North.

Negative or Westerly declination means that the needle of the compass will point West (or counterclockwise) of true North by however many degrees you got from the above calculation. The second diagram shows 10 W or -10 degrees on a compass. Wherever the needle of the compass points, it is 10 degrees West (or counterclockwise) of true North.

Now that you know this, you can put a mark on your compass, so you know where true North is depending on where the needle of the compass points. Some compasses have a wheel that you can turn, so all the markings are readjusted.

About Wood Trekker

I love the outdoors. I was born and raised in Bulgaria, but currently live in the US. I don’t know much about the woods, but I’m happy to share the little that I do know. If you have any questions or comments, please contact me at gilmore.ross@gmail.com.